Welcome.
In general, a reset *before* installing an update is not usually necessary.
When switching *between* firmware streams, from Asus to RMerlin and vice-versa, it is usually a really good idea to perform a full reset and then manually configure the router. Loading a saved configuration file in these circumstances is usually to be avoided since die to the way the data is stored, it essentially negates the point of the reset.
However, there is nothing wrong with simply loading the firmware version you want to try and see how it goes. But, if you run into anything strange after, then a reset and manual configure should be the first thing you do to see if it corrects the problem.
Word of advice, save all your old firmware versions, and save configuration files too. Keep the "sets" of firmware and configuration files together (personally, I rename the config file to include the firmware version info and date) in case you ever want to go back for whatever reason.